Sensing gas concentrations in Earth's lower atmosphere (e.g., the boundary layer) from an aircraft or an orbiting satellite/spacecraft equipped with passive sensors has been a formidable problem, especially for gases like carbon dioxide and methane whose detection is obscured by ambient amounts of gas in the upper atmosphere that mask the same gas in the boundary layer. The gas sensing task can be most difficult in the thermal infrared due to thermal contrast difficulties and water absorption. These difficulties are well-known in the art. Therefore, most passive sensor techniques rely on gas spectral features in the near infrared (i.e., wavelengths short of 3.5 microns) using scattered sunlight as the source. Attempts to make these measurements with various types of spectrometers have met with limited success since spectrally scanning spectrometer measurements of backscattered radiation from moving or spatially scanning instruments are difficult to interpret due to the highly variable nature of the scenes being imaged. The primary variability is the spectral and Bi-directional Reflectance Distribution Function (BRDF) character of the scattering surface. Static hyperspectral imaging instruments, such as Fabry-Perot spectrometers, overcome the spectral scanning problems, but face great difficulty in “stitching” together spectra from measurement samples as the scene locations pass through the spectrometer's field-of-view. Further, it is very difficult to calibrate and simulate the massive amounts of data in an analysis process. Still further, principal component analysis shows that sensitivity to the boundary layer is tenuous to begin with and is easily obscured by conditions of highly variable albedo and BRDF.
One method that addresses the complexity of spectra creation and calibration is a type of gas filtering commonly called Gas Filter Correlation Radiometry (GFCR). Briefly, in GFCR, a scene is viewed through gas cells having various amounts of a target gas (e.g., one cell filled with the target gas and one cell is empty). Spectral filtering is provided by the gas cell spectra and accurately-known cell gas content and conditions. The impact of albedo and BRDF variations will be the same for the multiple gas cell images regardless of gas cell condition, thereby nearly eliminating the error due to those effects. However, the only orbiting GFCR implementation to date with any success has been instrumentation that modulates the cell condition, which effectively modulates the sensitivity to only the target gas. Ideally, this is a major advantage. However, this implementation has two problems. First, the modulation induces a temporal change in filtering that can combine with the scene variability to create a temporal noise as the scene passes through the field-of-view. A second major problem is low sensitivity due to the inherently small modulation of the filtering function.